**4.4 Characterization of the gold nanoshells**

**Figure 13a** and **b** presents SEM and TEM images of the synthesized GNS, respectably. They illustrate that the silica particles are almost surrounded by gold. The higher density of gold, the separation of the GNPs [31], and the dielectric properties of the silica [3] contribute to the absorption of the NIR wavelength, which causes the increase in temperature.

### **4.5 The UV-Vis spectrum**

**Figure 14** presents the UV-Vis spectrum of the particles through the process. Silica particles, as well as functionalized silica particles, do not show significant

#### **Figure 14.**

*UV-Vis spectrum of (a) silica, (b) functionalized silica, (c) gold nanoparticles, (d) gold seeded silica, and (e) gold nanoshells.*

#### **Figure 15.**

*Thermography images of the GNS taken (a) before and (b) after being irradiated with 840 nm wavelength laser for 2 min.*

absorption on the NIR. As for the GNPs, they exhibit their characteristic absorption between 520 and 530 nm [29]. However, on the seeded silica particles, the slight shift to the NIR is noticeable. While on the GNS, the peak not only shifted to the NIR, but it kept a high absorbance all the way to 1100 nm. This range is part of the optical window of the human body [5]. The absorbance of the GNSs is due to the SPR that creates an electric field on the surface increasing the absorption of these wavelengths. SPR happens when metal nanoparticles are irradiated with a wavelength bigger than their size, exciting the electrons of the conducting band [2].

#### **4.6 Heat generation of GNS from light energy**

To verify the absorbance of the GNS, they were irradiated with an 840 nm laser with a power of 3.1 mW. **Figure 15a** and **b** displays the thermography images of the sample while being irradiated at time zero and 2 min later. The temperature of the sample increased from 24.7 to 31.0°C. This confirms that GNSs are able to absorb light from the NIR and convert it in heat.
